Ferroresonance requires a nonlinear inductive element, capacitance and a switching or connection condition that allows an unusual resonant circuit to form. A transformer alone does not imply ferroresonance. Screening should identify single-pole switching, lightly loaded voltage transformers or transformers, underground cable capacitance, ungrounded systems and other topologies that can create the required ingredients.
Nonlinearity makes ferroresonance different from a simple LC peak
A saturable magnetic element changes its effective inductive behavior as the voltage and flux evolve. Coupled with capacitance and a suitable connection, this can produce responses that a linear resonance calculation does not predict. Initial conditions and damping can affect which response develops. The presence of both a transformer and a cable is therefore not sufficient to declare ferroresonance, but a linear frequency scan alone is not sufficient to rule it out in a susceptible topology.
Look for the circuit created by an abnormal or partial connection
Screen single-pole switching states, an open phase, cable-fed lightly loaded transformers and voltage-transformer circuits where capacitance and magnetic nonlinearity can interact. Grounding and the connected load can change the available paths and damping. Draw the actual circuit for each credible state instead of listing equipment names as a diagnosis. The most relevant configuration may be a temporary or abnormal state that is absent from the normal one-line operating diagram.
Validate mitigation over the credible initial conditions
Possible measures depend on the identified circuit and may involve switching philosophy, grounding, damping or equipment selection. Their suitability must be checked with the affected suppliers and system engineer. A nonlinear transient model should retain reasonable initial flux, charge and switching variations rather than demonstrate only one favorable case. Report the limitations of the model and the operating states covered; a generic damping component or switching rule should not be prescribed without that application assessment.
Technical reference framework
[1] CIGRE Technical Brochure 568 — Transformer energization in power systems: A study guide.
[2] IEC 60076-8:1997 — Application guide.
[3] IEC 60071-2:2023 — Insulation co-ordination, Application guidelines.